You don't say where the party is being held of the time of day. Also, I don't know whether you are male or female. So, here I go!
If you are male and the engagement party is a formal one, possibly held in the evening, you would wear a dark suit and tie. The invitation should tell you whether the party required formal attire or not. Ask others that will be in attendance. If you know that the celebration is casual, again, the invitation should indicate this, you would wear nice pants, or possible a neat pair of jeans (no rips, tears, holes, etc.) and possibly a sports coat. You might or might not wear a tie. You could always wear one and tuck it away if you see that others are not wearing one.
If you are female and the celebration is a formal one, you might wear a formal cocktail dress that is fashionable, not gaudy or too short, possibly knee length or just below the knee. Black or some other solid colour that is dark would be preferable. Not too low cut or backless. If the party is casual, I would still suggest that the dress be knee length or slightly below the knee. Any colour of dress would be fine, but I would still steer clear of gaudy, multi coloured dresses. Choose something that is tastefully attractive i.e. a pantsuit with the pants not being too tight and a jacket that goes past the waist. Wear shoes that allow you to walk about and socialize with the other guests.
These would be my suggestions, but you could also "google" for other ideas and colours that would be appropriate. I tend to choose clothing that is more conservative than necessary. Use your good sense and enjoy yourself!